Due to the asynchronous behavior of the input framework and broker
communication, change handlers were previously called even after
zeek_done() event processing completed and also broker shutdown.
Accessing broker store handles within change handlers this late
triggered invalid Broker store handle messages:
error in ././my_option_store.zeek, line 13: invalid Broker store handle (Broker::put(Test::store, to_any_coercemy_option, to_any_coerceTest::new_value, 0 secs) and broker::store::{})
Fixes#2010
